Traditionally, there has been disclosed a catalytic reactor device to which a cassette accommodating a plurality of catalysts is detachably attached, the catalytic reactor device including a shelf board which is horizontally provided near a lower edge of an opening of a housing of the catalytic reactor device and on which the cassettes are placed and supported. In addition, the shelf board has a vent hole that is a rectangular opening, so that exhaust gas having entered the housing can pass through the cassette via the vent hole. (See Patent Literature 1 (PTL 1).)
With the configuration of this catalytic reactor device, the plurality of catalysts is fitted in the cassette, the cassette is charged onto the shelf board through the opening of the catalytic reactor, and then the opening is closed by a lid member, so that the cassette is fixed to the catalytic reactor device.